Engine system and gas fuel combustion method

ABSTRACT

The present disclosure describes an engine system that can achieve at least one of the followings: suppressing of generating of nitrogen oxides and suppressing of remaining of uncombusted hydrocarbons. The engine system has a combustion chamber to which air and a gas fuel are supplied, and is configured to combust the gas fuel. The engine system includes a liquid fuel injecting unit, and a control unit. The liquid fuel injecting unit is configured to inject a liquid fuel thereby to ignite the gas fuel. The control unit is configured to control the liquid fuel injecting unit. The control unit is configured to control the liquid fuel injecting unit so that injection of the liquid fuel is performed after a flame propagation after ignition of the gas fuel is ended.

TECHNICAL FIELD

The present invention relates to an engine system and a gas fuelcombustion method.

BACKGROUND ART

The internal combustion engine operating method described in PatentDocument 1 operates a diesel-type dual-fuel internal combustion engine.An internal combustion engine includes a combustion chamber, a firstfuel supply unit for a first fuel, and a second fuel supply unit for asecond fuel. The internal combustion engine operating method includessteps 1 through 4.

In the first step, the first fuel is premixed in the combustion chamber.In the second step, a charge material containing the first fuel iscompressed to a condition that allows the second fuel to auto-ignite. Inthe third step, a first injection of the second fuel into the combustionchamber is performed to start the auto-ignition of the second fuel,thereby igniting the first fuel. This starts the condition for premixedflame propagation combustion of the first fuel. In the fourth step, atleast one subsequent injection is performed, but the subsequentinjection provides an additional kinetic energy to a combustion step.

PRIOR ART DOCUMENT Patent Document

-   Patent Document 1: Japanese Unexamined Patent Application    Publication No. 2012-532273

SUMMARY OF INVENTION Technical Problem

However, the internal combustion engine operating method described inPatent Document 1 performs the subsequent injection so as to increasethe flame propagation rate. Increasing the flame propagation rateincreases the combustion rate, resulting in a higher maximum temperatureduring combustion. As a result, the generated volume of nitrogen oxidesincreases during the combustion of the first fuel. In addition,uncombusted hydrocarbons are, as the case may be, generated during thecombustion of the first fuel.

The present invention has been made in view of the above problem, and anobject thereof is to provide an engine system and a gas fuel combustionmethod that can achieve at least one of the followings when combustinggas fuel: suppressing of generating of nitrogen oxides and suppressingof remaining of uncombusted hydrocarbons.

Solution to Problem

According to one aspect of the present invention, an engine system has acombustion chamber to which air and a gas fuel are supplied, andcombusts the gas fuel. The engine system includes a liquid fuelinjecting unit, and a control unit. The liquid fuel injecting unitinjects a liquid fuel thereby to ignite the gas fuel. The control unitcontrols the liquid fuel injecting unit. The control unit controls theliquid fuel injecting unit so that the injecting of the liquid fuel isperformed after a flame propagation after the igniting of the gas fuelis ended.

According to another aspect of the present invention, a gas fuelcombustion method in an engine in which air and a gas fuel are suppliedto a combustion chamber thereby to combust the gas fuel includes: a stepof injecting a liquid fuel thereby to ignite the gas fuel; and a step ofperforming an injecting of the liquid fuel after a flame propagationafter the igniting of the gas fuel is ended.

Advantageous Effects of Invention

The present invention can provide an engine system and a gas fuelcombustion method that can achieve at least one of the followings whencombusting gas fuel: suppressing of generating of nitrogen oxides andsuppressing of remaining of uncombusted hydrocarbons.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing a configuration of an enginesystem according to an embodiment of the present invention.

FIG. 2 is a block diagram showing the engine system according to thepresent embodiment.

FIG. 3A is a diagram showing an example of liquid fuel injection timingaccording to the present embodiment. FIG. 3B is a diagram showinganother example of the liquid fuel injection timing according to thepresent embodiment.

FIG. 4 is a diagram schematically showing combustion characteristics ofgas fuel in an engine according to the present embodiment.

FIG. 5A is a graph schematically showing the remaining volume ofuncombusted hydrocarbons in the engine according to the presentembodiment. FIG. 5B is a graph schematically showing the generatedvolume of nitrogen oxides in the engine according to the presentembodiment.

FIG. 6 is a time chart schematically showing a combustion cycle of theengine according to the present embodiment.

FIG. 7 is a diagram schematically showing an injection angle of theliquid fuel according to the present embodiment.

FIG. 8 is a flowchart showing a gas fuel combustion method according tothe present embodiment.

FIG. 9 is a flowchart showing the gas fuel combustion method accordingto a modified example of the present embodiment.

FIG. 10A is a graph showing the remaining volume of uncombustedhydrocarbons in the engines according to examples 1 to 6 of the presentinvention. FIG. 10B is a graph showing the generated volume of nitrogenoxides in the engines according to examples 1 to 6 of the presentinvention.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will be described hereinafterwith reference to the accompanying drawings. Note that, in the drawings,the same reference signs are used for the same or equivalent components,and repeated descriptions are omitted.

Referring to FIGS. 1 through 8 , an engine system 100 of the embodimentof the present invention is described. First, the engine system 100 isdescribed with reference to FIG. 1 . FIG. 1 is a schematic diagramshowing a configuration of the engine system 100. The engine system 100shown in FIG. 1 combusts a gas fuel. Specifically, the engine system 100combusts the gas fuel thereby to obtain mechanical work. The gas fuel isnot particularly limited, examples thereof including hydrogen, ammonia,or natural gas. Natural gas is, for example, vaporized liquefied naturalgas (LNG). The engine system 100 is, for example, mounted in a vehicle,installed in a building, or installed outdoors. Vehicles are, forexample, vessels, automobiles, railcars, or airplanes.

The following is an example of a vessel 200 as a vehicle in which theengine system 100 is installed. In the present specification, the vessel200 may be read as “vehicle”.

As shown in FIG. 1 , the vessel 200 is equipped with the engine system100. The engine system 100 includes an engine 1, a gas fuel supply pipe3, a supercharger 5, an intercooler 7, an air supply pipe 9, an airsupply manifold 11, and an exhaust pipe 13.

The engine 1 combusts the gas fuel. Specifically, the engine 1 combuststhe gas fuel thereby to obtain mechanical work. The engine 1 is, forexample, a four-step engine. The engine 1 performs the combustion cyclerepeatedly. The combustion cycle includes an intake stroke, acompression stroke, a combustion stroke, and an exhaust stroke. Theengine 1 is, for example, an engine for propelling the vessel 200 or fordriving a generator.

The gas fuel supply pipe 3 supplies the gas fuel to the engine 1. Theair supply pipe 9 provides air from outside the engine 1 through thesupercharger 5, the intercooler 7, and the air supply manifold 11 to theengine 1. In other words, the air supply pipe 9 has an air supplypassage 91. The air then flows through the air supply passage 91 and theair is supplied to the engine 1.

Specifically, the supercharger 5 and the intercooler 7 are arranged inthis order, from upstream to downstream of the air supply. To the engine1, the supercharger 5 supplies air with a pressure greater thanatmospheric pressure. Specifically, the supercharger 5 compresses theair flowing through the air supply pipe 9, and the air with a pressuregreater than atmospheric pressure flows into the air supply pipe 9.Hereinafter, “compressed air” shows air at a pressure greater thanatmospheric pressure.

The intercooler 7 cools the air compressed by the supercharger 5 andsupplies the cooled air to the air supply manifold 11. The air supplymanifold 11 supplies the compressed and cooled air to the engine 1. Inother words, the air supply manifold 11 has an air supply passage 111.The compressed and cooled air is then supplied to the engine 1 via theair supply passage 111. Specifically, the engine 1 has a plurality ofcylinders 1 a. FIG. 1 shows one cylinder 1 a for simplicity of drawing.The air supply manifold 11 then supplies the compressed and cooled airto each cylinder 1 a. The engine 1 may have one cylinder 1 a. In thiscase, the air supply manifold 11 can be omitted.

Exhaust gas discharged from the engine 1 flows into the exhaust pipe 13.In other words, the exhaust pipe 13 discharges the exhaust gas to theoutside of the engine 1. Specifically, the exhaust pipe 13 has anexhaust passage 131. The exhaust gas then flows through the exhaustpassage 131 and the exhaust gas is discharged from the engine 1.

Exhaust gases are utilized by the supercharger 5. Specifically, thesupercharger 5 includes a turbine 51 and a compressor 52. The turbine 51is located in the exhaust pipe 13, and the compressor 52 is located inthe air supply pipe 9. The turbine 51 is rotated by the exhaust gasflowing through the exhaust pipe 13 and transfers rotational power tothe compressor 52. The compressor 52 is then driven by the rotationalforce of the turbine 51 thereby to compress the air flowing through theair supply pipe 9 thereby to generate air at a pressure greater thanatmospheric pressure.

The engine 1 has a cylinder head 61, a cylinder block 62, an air supplyvalve 63, an exhaust valve 64, a gas fuel supply unit 65, a liquid fuelinjecting unit 66, a liner 67, a piston 68, a connecting rod 69, and acrankshaft 70. The engine 1 also has a combustion chamber 71. Thecombustion chamber 71 is formed inside the cylinder block 62, and is aspace where the gas fuel is combusted. Air and gas fuel are supplied tothe combustion chamber 71. In other words, the engine 1 supplies air andgas fuel to the combustion chamber 71 thereby to combust the gas fuel.

The cylinder head 61 is fixed to the top of cylinder block 62. Thecylinder head 61 has an air supply passage 72 and an exhaust passage 73.

The air supply manifold 11 is connected to an inlet of the air supplypassage 72. Thus, the compressed and cooled air is supplied to the airsupply passage 72 from the air supply passage 111 of the air supplymanifold 11. An outlet of the air supply passage 72 is connected to thecombustion chamber 71.

The gas fuel supply unit 65 is located in the cylinder head 61. Then, tothe air supply passage 72, the gas fuel supply unit 65 supplies the gasfuel supplied from the gas fuel supply pipe 3, to thereby supply the gasfuel to the combustion chamber 71 through the air supply passage 72. Forexample, the gas fuel supply unit 65 provides the gas fuel to the airsupply passage 72.

Specifically, the gas fuel is mixed with the air supplied from the airsupply passage 111 of the air supply manifold 11 and is then supplied tothe combustion chamber 71. In other words, a mixture of gas fuel and airis supplied to the combustion chamber 71. The gas fuel supply unit 65is, for example, a gas admission valve (GAV) or a gas injector. Themixed gas is preferably a lean mixture. In this case, the engine 1combusts the gas fuel in the lean mixture.

More specifically, the air supply valve 63 is located at the outlet ofthe air supply passage 72. The air supply valve 63 opens or closes theoutlet of the air supply passage 72. When the air supply valve 63 opensthe outlet of air supply passage 72, a mixture of gas fuel and air issupplied to the combustion chamber 71. In detail, when the air supplyvalve 63 opens, the gas fuel supply unit 65 injects the gas fuel intothe air supply passage 72 thereby to supply the gas fuel via the airsupply passage 72 to the combustion chamber 71.

For example, the gas fuel supply unit 65 may be located in the airsupply manifold 11 or may be located in the air supply pipe 9 downstreamfrom the intercooler 7.

The inlet of the exhaust passage 73 is connected to combustion chamber71. The outlet of exhaust passage 73 is connected to exhaust pipe 13.Thus, the exhaust gas from combustion chamber 71 is discharged throughthe exhaust passage 73 to the exhaust pipe 13. Specifically, the exhaustvalve 64 is placed at the inlet of the exhaust passage 73. The exhaustvalve 64 opens or closes the inlet of the exhaust passage 73. Theexhaust valve 64, when opening the inlet of the exhaust passage 73,discharges the exhaust gas through the exhaust passage 73 to the exhaustpipe 13.

The liquid fuel injecting unit 66 injects the liquid fuel, which inducesignition of the gas fuel, into the combustion chamber 71. In otherwords, the liquid fuel injecting unit 66 injects the liquid fuel intothe combustion chamber 71 thereby to ignite the gas fuel supplied to thecombustion chamber 71. The injection volume of the liquid fuel by theliquid fuel injecting unit 66 is small enough to induce ignition of thegas fuel. The liquid fuel is, for example, light or heavy oil. Theliquid fuel injecting unit 66 is, for example, an injector.

The cylinder block 62 constitutes a cylinder 1 a. The cylinder block 62houses a piston 68, a connecting rod 69, and a crankshaft 70.

The liner 67 is a cylindrical body that fits into the cylinder block 62.The piston 68 reciprocates up and down inside the cylinder block 62along the liner 67. In other words, the piston 68 reciprocates withinthe combustion chamber 71. The connecting rod 69 connects the piston 68to the crankshaft 70. The connecting rod 69 transmits the reciprocatingmotion of the piston 68 to the crankshaft 70. The crankshaft 70 convertsthe reciprocating motion of the piston 68 into a rotational motion.

For example, when the piston 68 is lowered and the air supply valve 63is opened with the exhaust valve 64 closed, the mixture of gas fuel andair is supplied from the air supply passage 72 to the combustion chamber71 (intake stroke). In other words, the gas fuel supply unit 65 injectsthe gas fuel at the timing when the air supply valve 63 opens. Next,with the exhaust valve 64 and the air supply valve 63 closed, the piston68 rises (compression stroke). Next, at the top dead center of thepiston 68, the liquid fuel is injected by liquid fuel injecting unit 66,and the gas fuel is ignited and combusted (combustion stroke). As aresult, the combustion lowers the piston 68. Next, the piston 68 rises,and the exhaust valve 64 opens while the air supply valve 63 is closed(exhaust stroke). As a result, the exhaust gas is discharged from thecombustion chamber 71 to the exhaust passage 73.

As described above with reference to FIG. 1 , the engine 1 generatespower by combusting the gas fuel mixed with air.

Next, the engine system 100 is described with reference to FIG. 2 . FIG.2 is a block diagram showing the engine system 100. As shown in FIGS. 3Aand 3B, the engine system 100 is further equipped with an engine controlunit 21, an operation control unit 23. A measuring unit 25 is describedbelow as a modified example.

The operation control unit 23 receives an operation from a humanoperator and outputs an operation signal to the engine control unit 21in response to the operation from the human operator.

The operation control unit 23 includes, for example, an input unit, adisplay unit, and a computer. Input units include, for example,keyboards, pointing devices, dials, and push buttons. The display unitis, for example, a liquid crystal display. The display unit may include,for example, a touch screen. The computer includes, for example, aprocessor and a storage device. The operation control unit 23 is, forexample, an operation control panel.

The engine control unit 21 controls the engine 1. For example, theengine control unit 21 controls the engine 1 in response to an operationsignal output by the operation control unit 23. For example, the enginecontrol unit 21 controls the engine 1 according to a computer program.

The engine control unit 21 is, for example, a computer. Computers canbe, for example, ECUs (Electronic Control Units). Specifically, theengine control unit 21 includes a control unit 211 and a storage unit212. The control unit 211 includes a processor such as a CPU (centralprocessing unit). The storage unit 212 includes a storage device andstores data and a computer program. The storage devices include, forexample, main and auxiliary storage units such as semiconductor memory.The storage device may include a removable medium.

The control unit 211 controls the engine 1. Specifically, the controlunit 211 controls the gas fuel supply unit 65 and the liquid fuelinjecting unit 66. More specifically, the processor of the control unit211 executes the computer program stored in the storage device of thestorage unit 212, thereby to control the gas fuel supply unit 65 and theliquid fuel injecting unit 66.

In particular, in the present embodiment; so as to ignite the gas fuel,the control unit 211 controls the liquid fuel injecting unit 66 in amanner to inject the liquid fuel into the combustion chamber 71. As aresult, the liquid fuel injecting unit 66 injects the liquid fuel intothe combustion chamber 71. Thus, the gas fuel is ignited in thecombustion chamber 71, thereby to combust the gas fuel.

Further, the control unit 211 controls the liquid fuel injecting unit 66in a manner to execute the injection of liquid fuel after the flamepropagation after the ignition of the gas fuel is ended. As a result,the liquid fuel injecting unit 66 performs the injection of the liquidfuel after the flame propagation after the ignition of gas fuel isended. Thus, with the engine system 100 according to the presentembodiment, at least one of the followings can be achieved whencombusting the gas fuel: suppressing of generating of nitrogen oxidesand suppressing of remaining of uncombusted hydrocarbons. This isdemonstrated by examples described below. For example, when the mixtureof gas fuel and air (i.e., the mixture supplied to the combustionchamber 71) is in an effective lean state, igniting the liquid fuelafter the flame propagation after ignition of the gas fuel is ended cansuppress generating of nitrogen oxides and suppress remaining ofuncombusted hydrocarbons. This point is also demonstrated in theexamples described below.

The effective lean state is a state in which an air excess ratio λ ofthe mixed gas is within the specified range. The specified range of theair excess ratio λ is greater than “1” and is based on the engine 1'sspecifications. The specified range of the air excess ratio λ shows thepractically effective range of the engine 1. For example, the specifiedrange of the air excess ratio λ is between 1.80 and 2.00, bothinclusive.

In the present embodiment, preferably, the control unit 211 controls theliquid fuel injecting unit 66 in a manner to perform the liquid fuelinjection after the end of flame propagation, after the top dead centerof the piston 68, at the crank angle in the range of 30 degrees or moreand less than 60 degrees. As a result, the liquid fuel injecting unit 66injects the liquid fuel after the end of flame propagation, after thetop dead center of the piston 68, at the crank angle in the range of 30degrees or more and less than 60 degrees. Thus, according to the presentpreferred example, at least one of the followings can be achieved whencombusting the gas fuel: further suppressing of generating of nitrogenoxides and further suppressing of remaining of uncombusted hydrocarbons.This is demonstrated by examples described below. For example, when themixed gas is in an effective lean state, performing the liquid fuelinjection after the end of flame propagation, after the top dead centerof the piston 68, at the crank angle in the range of 30 degrees or moreand less than 60 degrees, can further suppress generating of nitrogenoxides and can further suppress remaining of uncombusted hydrocarbons.This point is also demonstrated in the examples described below. At thecrank angle in the range of 30 degrees or more and less than 60 degrees,the combustion chamber 71 is in a state where the flame propagationafter ignition of gas fuel has been ended.

In the following description, “injection of liquid fuel for ignition”is, as the case may be, described as “main injection”. The “injection ofliquid fuel after the end of flame propagation” is the injection ofliquid fuel following the “main injection”. Therefore, “injection ofliquid fuel after the end of flame propagation” are, as the case may be,described as “subsequent injection of liquid fuel”, “subsequentinjection”, or “subsequent injection”. However, “subsequent” is notlimited to “next” to the “main injection” as long as being “after” the“main injection,” but also includes further “next” to “next” to the“main injection,” for example. In other words, as long as being “after”the “main injection”, “subsequent” is “subsequent”. In the presentspecification, “subsequent” shows after the “main injection” and afterthe end of flame propagation.

Next, the main and subsequent injections by the liquid fuel injectingunit 66 are described with reference to FIGS. 2 and 3 . FIG. 3A is adiagram showing an example of liquid fuel injection timing by the liquidfuel injecting unit 66. The abscissa shows the crank angle. The crankangle is the phase angle of the piston 68. The abscissa can also beviewed as showing time by the crank angle.

As shown in FIG. 2 and FIG. 3A, the liquid fuel injecting unit 66performs the main injection during a predetermined main injection periodP0, and ends the main injection at the expiration of the predeterminedmain injection period P0. In the example in FIG. 3A, the liquid fuelinjecting unit 66 performs a main injection J0 before the top deadcenter of the piston 68.

Further, with a time interval P01 provided for the main injection J0,the liquid fuel injecting unit 66 executes a subsequent injection J1after the flame propagation is ended. The liquid fuel injecting unit 66performs the subsequent injection J1 in a predetermined subsequentinjection period P1, and ends the subsequent injection at the expirationof the predetermined subsequent injection period P1.

FIG. 3B is a diagram showing another example of the liquid fuelinjection timing by the liquid fuel injecting unit 66. As shown in FIGS.2 and 3B, the liquid fuel injecting unit 66 may perform a plurality ofsubsequent injections Jn with a time interval P. In the presentspecification, “n” at the end of a reference sign shows an integergreater than or equal to one. When a plurality of subsequent injectionsJn are performed, a plurality of predetermined subsequent injectionperiods Pn are set for each of the plurality of subsequent injectionsJn. In this case, the plurality of predetermined subsequent injectionperiods Pn may be the same or different. When three or more subsequentinjections Jn are performed, the time interval P of the temporallyadjacent subsequent injections Jn may be the same or may be different.

The subsequent injection Jn shows being an injection subsequent to themain injection J0. However, the subsequent injection Jn may be anyinjection after the main injection J0, and includes not only theinjection J1 immediately after the main injection J0, but alsoinjections J2, J3 . . . .

One or more injections of liquid fuel may be performed between the maininjection J0 and the subsequent injection J1 that is after the end offlame propagation.

Next, combustion characteristics of gas fuel, such as flame propagation,will be described, referring to FIG. 4 . FIG. 4 is a diagramschematically showing combustion characteristics of gas fuel in thecombustion chamber 71. In FIG. 4 , the abscissa shows the crank angle(deg. aTDC: after Top Dead Center). In FIG. 4 , the crank angle of 0degree shows the top dead center of the piston 68. The abscissa can alsobe viewed as showing time by the crank angle. The left ordinate shows anin-cylinder pressure (a. u.). The in-cylinder pressure shows theinternal pressure of the combustion chamber 71. The unit of thein-cylinder pressure is, for example, “MPa”. The right ordinate showsthe heat generation rate (a. u.) in the combustion chamber 71. The heatgeneration rate shows the combusting state of gas fuel in the combustionchamber 71. The unit of the heat generation rate is, for example,“J/deg.”.

As shown in FIG. 4 , a heat generation rate curve A1 shows the heatgeneration rate. A pressure curve A2 shows the in-cylinder pressure. InFIG. 4 , the relation among the heat generation rate curve A1, thepressure curve A2 and the crank angle is merely an exemplification. Theshapes of the heat generation rate curve A1 and pressure curve A2 arealso merely exemplifications. In the following, the heat generation ratecurve A1 attracts attention.

The main injection starts at a timing prior to the top dead center ofthe piston 68. In the example in FIG. 4 , the main injection is startedwhen the crank angle is “−20 degrees”.

Next, the start of gas fuel combustion is when the heat generation rate,shown by the heat generation rate curve A1, rises from zero(substantially zero). In the example in FIG. 4 , when the crank angle is“−10 degrees”, the gas fuel is ignited by the liquid fuel, startingcombustion of the gas fuel. The flame propagation starts in thecombustion chamber 71 at the same time when combustion of the gas fuelstarts in the combustion chamber 71.

Then, the end of flame propagation is generally when the heat generationrate, as shown by the heat generation rate curve A1, shows a maximumvalue. In the example in FIG. 4 , the flame propagation ends when thecrank angle is “10 degrees”. The end of flame propagation shows when theflame reaches an inner peripheral face of the combustion chamber 71.

A flame propagation period T0 shows a period from the start of flamepropagation (e.g., crank angle=−10 degrees) to the end of flamepropagation (e.g., crank angle=10 degrees).

The former half of the flame propagation period T0 is the period fromthe start of the flame propagation (e.g., crank angle=−10 degrees) tothe middle of the flame propagation (e.g., crank angle=0 degree). Thelatter half of the flame propagation period T0 is the period from themiddle of the flame propagation (e.g., crank angle=0 degree) to the endof flame propagation (e.g., crank angle=10 degrees).

The end of gas fuel combustion is when the heat generation rate, asshown by the heat generation rate curve A1, goes from a value greaterthan zero (substantially zero) to zero (substantially zero). In theexample in FIG. 4 , the gas fuel combustion ends when the crank angle is“50 degrees”.

A combustion period T12 of gas fuel is the period from the start of gasfuel combustion (e.g., crank angle=−10 degrees) to the end of combustion(e.g., crank angle=50 degrees). A period T1, which is the former half ofthe combustion period T12 of gas fuel, shows the period from the startof gas fuel combustion (e.g., crank angle=−10 degrees) to the middle ofthe combustion (e.g., crank angle=20 degrees). A period T2 of the latterhalf of the combustion period T12 of gas fuel shows the period from themiddle of gas fuel combustion (e.g., crank angle=20 degrees) to the endof the combustion (e.g., crank angle=50 degrees).

In the present embodiment, after the flame propagation after ignition ofthe gas fuel is ended, the liquid fuel injecting unit 66 (FIG. 1 )performs the liquid fuel injection subsequent to the main injection(subsequent injection). In other words, at and after the end of flamepropagation (10 degrees) and before the end of gas fuel combustion (50degrees), the liquid fuel injecting unit 66 performs the liquid fuelinjection subsequent to the main injection. The term “at and after theend” includes “at the end”.

Preferably, the control unit 211 (FIG. 2 ) controls the liquid fuelinjecting unit 66 so that the subsequent injection of liquid fuel(injection of liquid fuel after the end of flame propagation) startswithin a former period T3 of the combustion period T12 of gas fuel. As aresult, after the flame propagation after the ignition of the gas fuelis ended, the liquid fuel injecting unit 66 performs the subsequentinjection of the gas fuel within the former period T3 of the combustionperiod T12 of gas fuel. Thus, according to the present preferredexample, the temperature in the combustion chamber 71 is higher than atthe time of ignition, at the timing when the subsequent liquid fuelreaches the vicinity of the clevis volume. Thus, uncombustedhydrocarbons that leak from the clevis volume as the piston 68 movesfrom the top dead center to the bottom dead center can be effectivelyoxidized. As a result, the remaining volume of uncombusted hydrocarbonscan be effectively reduced. The period T3 shows the period when theflame propagation after the ignition of gas fuel is ended.

The clevis volume is a gap or a gap volume between the piston 68 and theliner 67.

Next, the generated volume of nitrogen oxides and the remaining volumeof uncombusted hydrocarbons according to the present embodiment will bedescribed referring to FIGS. 5A and 5B. FIG. 5A is a graph schematicallyshowing the remaining volume of uncombusted hydrocarbons in the engine1. The ordinate shows the remaining volume of uncombusted hydrocarbons(ppmC1). FIG. 5B is a graph schematically showing the generated volumeof nitrogen oxides in the engine 1. The ordinate shows the generatedvolume (ppm) of nitrogen oxides. In FIGS. 5A and 5B, the abscissa showsthe timing of the subsequent injection by the crank angle (deg. aTDC).The crank angle of 0 degree shows the top dead center of the piston 68.The abscissa can also be viewed as showing time by the crank angle.

In FIG. 5A, a dashed line L1 shows the remaining volume of uncombustedhydrocarbons that is seen when no subsequent injection is performed(main injection only, reference case). A dot D1, shown by a blackcircle, shows the remaining volume of uncombusted hydrocarbons that isseen when the subsequent injection is performed. Hereafter, theremaining volume that is seen when the subsequent injection is notperformed is, as the case may be, described as “L1,” and the remainingvolume that is seen when the subsequent injection is performed is, asthe case may be, described as “D1” corresponding to the dot D1.

The remaining volume D1 of uncombusted hydrocarbons that is seen whenthe subsequent injection is performed is less than the remaining volumeL1 of uncombusted hydrogen that is seen when the subsequent injection isnot performed. In particular, the larger the crank angle at which thesubsequent injection is performed, the smaller the remaining volume D1of uncombusted hydrocarbons. In other words, after the flame propagationafter the ignition of gas fuel is ended, the later the timing of thesubsequent injection, the less the remaining volume D1 of uncombustedhydrocarbons.

In FIG. 5B, a dashed line L2 shows the generated volume of nitrogenoxides that is seen when the subsequent injection is not performed (maininjection only, reference case). A dot D2, shown by a black circle,shows the generated volume of nitrogen oxides that is seen when thesubsequent injection is performed. Hereafter, the generated volume thatis seen when the subsequent injection is not performed is, as the casemay be, described as “L2,” and the generated volume that is seen whenthe subsequent injection is performed is, as the case may be, describedas “D2” corresponding to the dot D2.

The generated volume D2 of nitrogen oxides that is seen when thesubsequent injection is performed is less than a generated volume L2 ofnitrogen oxides that is seen when the subsequent injection is notperformed. In particular, the smaller the crank angle at which thesubsequent injection is performed, the less the generated volume L2 ofnitrogen oxides. In other words, after the flame propagation after theignition of gas fuel is ended, the earlier the timing of the subsequentinjection, the less the generated volume L2 of nitrogen oxides.

As shown in FIGS. 5A and 5B, performing the subsequent injection canreduce the remaining volume of uncombusted hydrocarbons and thegenerated volume of nitrogen oxides. However, the subsequent injection'stiming when a reduction width (=L1−D1) of the remaining volume D1 ofuncombusted hydrocarbons becomes large (e.g., crank angle=60°) is thesame as the subsequent injection's timing (e.g., crank angle=60 degrees)when a reduction width (=L2−D2) of the generated volume D2 of nitrogenoxides becomes small. Meanwhile, the subsequent injection' timing whenthe reduction width of the generated volume D2 of nitrogen oxidesbecomes large (e.g., crank angle=30 degrees) is the subsequentinjection's timing (e.g., crank angle=30 degrees) when the reductionwidth of the remaining volume D1 of uncombusted hydrocarbons becomessmall. Therefore, adjusting the timing of the subsequent injection canadjust the remaining volume D1 of uncombusted hydrocarbons and thegenerated volume D2 of nitrogen oxides.

The uncombusted hydrocarbons and the nitrogen oxides are one example ofexhaust substances from engine 1.

In particular, in the present embodiment, it is preferable that thecontrol unit 211 (FIG. 2 ) changes at least one of the followings: thenumber of subsequent injections of liquid fuel (the number of injectionsof liquid fuel after the end of flame propagation), the volume ofsubsequent injections of liquid fuel (the injection volume of liquidfuel after the end of flame propagation), and the timing of thesubsequent injection of liquid fuel (timing of the injection of theliquid fuel after the end of flame propagation), thereby to adjust thevolume of exhaust substances (uncombusted hydrocarbons and nitrogenoxides) from the engine 1. According to the present preferred example,while reducing the remaining volume of uncombusted hydrocarbons and thegenerated volume of nitrogen oxides compared to the case in which thesubsequent injection is not performed, the balance between the remainingvolume of uncombusted hydrocarbons and the generated volume of nitrogenoxides can be easily adjusted.

Specifically, based on the relation between the remaining volume D1 ofuncombusted hydrocarbons and the timing of the subsequent injection, andthe relation between the generated volume D2 of nitrogen oxides and thetiming of subsequent injection, the control unit 211 changes at leastone of the followings: the number of subsequent injections, theinjection volume of the subsequent injection, and the timing of thesubsequent injection, thereby to adjust the remaining volume ofuncombusted hydrocarbons and the generated volume of nitrogen oxides. Inthis case, the control unit 211 can change the number of subsequentinjections, the injection volume of the subsequent injections, and thetiming of the subsequent injections by controlling the liquid fuelinjecting unit 66.

As an example, the control unit 211 performs the subsequent injectionsat both of the subsequent injection's timing (e.g., crank angle=30degrees) when the reduction width (=L2−D2) of the generated volume D2 ofnitrogen oxides is large, and the subsequent injection's timing (e.g.,crank angle=60 degrees) when the reduction width (=L1−D1) of theremaining volume D1 of uncombusted hydrocarbons is large. In thisexample, both of the reduction width of the generated volume D2 ofnitrogen oxides, and the reduction width of the remaining volume D1 ofuncombusted hydrocarbons can be increased.

Referring to FIGS. 5A and 5B continuously, the relation between theuncombusted hydrocarbons and the nitrogen oxides, and the air excessratio λ and the air-fuel ratio is described.

The air excess ratio λ is a value obtained by dividing a mass Ma of airactually supplied to the combustion chamber 71 by a theoreticallyrequired minimum mass Mb of air (Ma/Mb), and is an indicator of thedegree of air surplus in the mixed gas. The air excess ratio λ is alsoequal to the actual air-fuel ratio divided by the theoretical air-fuelratio. For example, when the air excess ratio λ is less than “1”, themixed gas of gas fuel and air is a rich mixture (a mixture of dense gasfuel). The state in which the air excess ratio λ of the mixed gas isless than “1” is, as the case may be, described as a “rich state”.Meanwhile, when, for example, the air excess ratio λ is greater than“1”, the mixed gas of gas fuel and air is a lean mixture (a mixture ofthin gas fuel). The state in which the air excess ratio λ of the mixedgas is greater than “1” is, as the case may be, described as a “leanstate”.

The air-fuel ratio is a value obtained by dividing a mass Mx of air by amass My of gas fuel (Mx/My).

In FIG. 5A, a dot D10, shown by a white circle, shows the remainingvolume of uncombusted hydrocarbons that is seen when the subsequentinjection is performed. Hereafter, the remaining volume of uncombustedhydrocarbons that is seen when the subsequent injection is performed is,as the case may be, referred to as “D10” corresponding to the dot D10.

The air excess ratio λ (e.g., lean state richer than the effective leanstate) seen when the remaining volume of uncombusted hydrocarbons showsthe remaining volume D10 is smaller than the air excess ratio λ (e.g.,the effective lean state) seen when the remaining volume of uncombustedhydrocarbons shows the remaining volume D1. Therefore, when the timingof the subsequent injection is the same, the smaller the air excessratio λ (air-fuel ratio), the less the remaining volume of uncombustedhydrocarbons. Therefore, controlling the air excess ratio λ (air-fuelratio) can adjust the remaining volume of uncombusted hydrocarbons, evenwhen the timing of the subsequent injection is the same.

In FIG. 5B, a dot D20, shown by a white circle, shows the generatedvolume of nitrogen oxides that is seen when the subsequent injection isperformed. Hereafter, the generated volume of nitrogen oxides that isseen when the subsequent injection is performed is, as the case may be,referred to as “D20” corresponding to the dot D20.

The air excess ratio λ (e.g., the lean state richer than the effectivelean state) seen when the generated volume of nitrogen oxides shows thegenerated volume D20 is smaller than the air excess ratio λ (e.g., theeffective lean state) seen when the generated volume of nitrogen oxidesshows a generated volume D2. Therefore, when the timing of thesubsequent injection is the same, the smaller the air excess ratio λ(air-fuel ratio), the more the generated volume of nitrogen oxides.Therefore, controlling the air excess ratio λ (air-fuel ratio) canadjust the generated volume of nitrogen oxides, even when the timing ofthe subsequent injection is the same.

In particular, in the present embodiment, the control unit 211 (FIG. 2 )controls the air excess ratio λ in the combustion chamber 71 accordingto the condition for injecting the subsequent liquid fuel (condition forsubsequent injection). In other words, the control unit 211 controls theair excess ratio λ in the combustion chamber 71 according to thecondition for injecting liquid fuel after the end of flame propagation.As a result, the balance between the effect of reducing the remainingvolume of uncombusted hydrocarbons and the effect of reducing thegenerated volume of nitrogen oxides can be adjusted according to theobject of reducing the exhaust substance.

The condition for the subsequent injection is, for example, the timingof the subsequent injection. As can be understood from FIGS. 5A and 5B,depending on the timing of the subsequent injection, to which reductioneffect, that is, the effect of reducing uncombusted hydrocarbons and theeffect of reducing nitrogen oxides the subsequent injection contributesvaries. So, for example, when the subsequent injection is performed atthe crank angle with a great reduction volume of nitrogen oxides (e.g.,timing of 30 degree), the control unit 211 shifts the air excess ratio λto the rich side while maintaining the air excess ratio λ lean. As aresult, the generated volume of nitrogen oxides, which had cleared thelegal limit level with a margin, can be returned to the legal limitlevel, making it possible to increase the effect of reducing theremaining volume of uncombusted hydrocarbons.

Here, the control quantity for controlling the air excess ratio λ is,for example, the mixed gas flow rate, gas fuel flow rate, or air flowrate. Therefore, the control unit 211 can control the air excess ratio λby controlling the control amount.

Next, a combustion cycle BCY of the engine 1 is described with referenceto FIGS. 2 and 6 . FIG. 6 is a time chart schematically showing thecombustion cycle BCY of the engine 1. The abscissa shows time. Time isshown, for example, by the crank angle. A pulse shape Cl schematicallyshows the period of supplying the gas fuel. A curve A1 corresponds tothe heat generation rate curve A1 in FIG. 4 , and schematically showsthe heat generation state. A period T0 corresponds to the flamepropagation period T0 in FIG. 4 , and a period T12 corresponds to thecombustion period T12 in FIG. 4 .

As shown in FIG. 2 and FIG. 6 , the engine 1 repeats the combustioncycle BCY. A predetermined period T attracts attention. The engine 1performs a plurality of combustion cycles BCY in the predeterminedperiod T. For each combustion cycle BCY, the liquid fuel injecting unit66 performs ignition of gas fuel (main injection) and the subsequentinjection of liquid fuel (injection of liquid fuel after the end offlame propagation). Within the predetermined period T, the control unit211 prohibits an increase in the liquid fuel's injection volume(injection volume of the main injection) that is seen when igniting thegas fuel. The “increase in the liquid fuel's injection volume that isseen when igniting the gas fuel” refers to an increase in the injectionvolume of liquid fuel that corresponds to an increase in a load on theengine 1. The increase in load shows, for example, that the load on theengine 1 exceeds a threshold value. The cause for the load increase maybe, for example, an increase in vessel speed, a change in rudder angle,or a strong wave or wind to which the vessel is subjected. Thepredetermined period T shows 10 seconds.

In other words, in the present embodiment, the increase (within 10seconds), due to the increase in load, in the liquid fuel's injectionvolume that is seen when igniting the gas fuel (injection volume of maininjection) is prohibited. In other words, a short-term increase ininjection volume caused by the increase in load on the engine 1 isprohibited.

Thus, in the engine system 100 having the supercharger 5, the injectionof liquid fuel to such an extent as to accompany a sudden increase inair feed pressure is prohibited. As a result, the air excess ratio λ(air-fuel ratio) can be suppressed from causing a large fluctuationrelative to a target value, making it possible to prevent an emissionfrom deteriorating. Deterioration of the emission shows, for example, anincrease in generated volume of nitrogen oxides or an increase in theremaining volume of uncombusted hydrocarbons.

In FIG. 6 , the flame propagation period T0 is, for example, about 4.6milliseconds (=about 20 degrees crank angle). The combustion period T12of gas fuel is, for example, about 13.4 milliseconds (=crank angle ofabout 60 degrees). The combustion cycle BCY is, for example, ⅙ second(=720 degree crank angle).

Next, the liquid fuel's injection angle θ by the liquid fuel injectingunit 66 is described with reference to FIG. 7 . FIG. 7 is a diagramschematically showing the injection angle θ of the liquid fuel accordingto the present embodiment. As shown in FIG. 7 , an uncombustedhydrocarbon THC that had been pushed into a clevis volume VL leaks outof the clevis volume VL when the piston 68 moves from the top deadcenter to the bottom dead center.

Then, in the present embodiment, the liquid fuel's injection angle θ bythe liquid fuel injecting unit 66, that is, the liquid fuel's injectionangle θ relative to the direction of movement of the piston 68 isbetween 30 degrees and 65 degrees, both inclusive. Therefore, at thetiming when the uncombusted hydrocarbon THC leaks out of the clevisvolume VL when the piston 68 moves from the top dead center to thebottom dead center, the liquid fuel injected from the liquid fuelinjecting unit 66 (e.g., liquid fuel by subsequent injection) reachesthe vicinity of the clevis volume VL. As a result, the liquid fuel caneffectively oxidize the uncombusted hydrocarbon THC leaking out of theclevis volume VL. Thus, the remaining volume of uncombusted hydrocarbonTHC can be effectively reduced.

The control unit 211 (FIG. 2 ) may, depending on specifications for theengine 1 and/or specifications for the liquid fuel injecting unit 66,set a limitation on at least one of the control of the injection volumeof liquid fuel in the subsequent injection and the control of the timingof the subsequent injection. This is because, depending on the injectionvolume of liquid fuel in the subsequent injection and/or the timing ofthe subsequent injection, there may be a possibility that the liquidfuel should impinge on the liner 67 thereby to dilute the lubricatingoil on the liner 67 surface by the liquid fuel. The specifications forthe engine 1 are, for example, the bore or stroke of the engine 1. Thespecifications for the liquid fuel injecting unit 66 are, for example,the injection diameter, injection angle θ, or injection volume of theliquid fuel injecting unit 66.

Referring now to FIGS. 2 and 8 , the gas fuel combustion method in theengine 1 is described. FIG. 8 is a flowchart showing the gas fuelcombustion method according to the present embodiment. The gas fuelcombustion method is performed by the engine system 100 shown in FIG. 2. As shown in FIG. 8 , the gas fuel combustion method includes steps S1through S4.

First, in step S1, the control unit 211 of the engine system 100determines whether or not the timing for the main injection has arrived.

When it is determined in step S1 that the timing for the main injectionhas not arrived (No), the process waits for step S1.

Meanwhile, when it is determined in step S1 that the timing for the maininjection has arrived (Yes), the process proceeds to step S2.

Next, in step S2, the control unit 211 controls the liquid fuelinjecting unit 66 in a manner to perform the main injection. As aresult, the liquid fuel injecting unit 66 performs the main injection.In other words, the liquid fuel injecting unit 66 injects the liquidfuel thereby to ignite the gas fuel. As a result, gas fuel is combusted.

Next, in step S3, the control unit 211 determines whether or not thetiming for the subsequent injection has arrived.

When it is determined in step S3 that the timing for the subsequentinjection has not arrived (No), the process waits for step S3.

Meanwhile, when it is determined in step S3 that the timing for thesubsequent injection has arrived (Yes), the process proceeds to step S4.

Next, in step S4, the control unit 211 controls the liquid fuelinjecting unit 66 to in a manner to execute the subsequent injection. Asa result, the liquid fuel injecting unit 66 performs the subsequentinjection. Specifically, after the flame propagation after the ignitionof gas fuel is ended, the liquid fuel injecting unit 66 executes theinjection of liquid fuel. Thereafter, the process proceeds to step S1.

Steps S1 through S4 are executed in one combustion cycle. Thus, steps S1through S4 are executed for each repeated combustion cycle. In otherwords, steps S1 through S4 are repeated.

As described above with reference to FIG. 8 , with the gas fuelcombustion method according to the present embodiment, the injection ofliquid fuel (subsequent injection) is executed after the flamepropagation after the ignition of gas fuel is ended. As a result, atleast one of the followings can be achieved: suppressing of generatingof nitrogen oxides and suppressing of remaining of uncombustedhydrocarbons. In particular, when the mixed gas supplied to thecombustion chamber 71 is in the effective lean state, the generating ofnitrogen oxides can be suppressed while the remaining of the uncombustedhydrocarbons can be suppressed.

When the liquid fuel injecting unit 66 executes a plurality ofsubsequent injections in one combustion cycle, the same processes as instep S3 and step S4 are executed a plurality of times.

Modified Example

Referring to FIG. 2 and FIG. 9 , the engine system 100 according to amodified example of the present embodiment is described. The modifiedexample mainly differs from the present embodiment described above inthat the engine system 100 according to the modified example is equippedwith a measuring unit 25. Mainly described below are points in which themodified example differs from the present embodiment.

As shown in FIG. 2 , in the modified example, the engine system 100 isfurther equipped with the measuring unit 25. The measuring unit 25measures a physical quantity that directly or indirectly shows theexhaust substance resulting from the combustion of gas fuel. Then, basedon a measurement result of the measuring unit 25, the control unit 211controls the liquid fuel's subsequent injection by the liquid fuelinjecting unit 66 (injection of liquid fuel after the end of flamepropagation). In other words, in the modified example, feedback controlof subsequent injection is executed according to the measurement resultof the physical quantity that directly or indirectly shows the exhaustsubstance. Thus, the subsequent injection can be optimized according tothe combustion state of the gas fuel. As a result, the volume of exhaustsubstance can be effectively reduced. In other words, while generatingof nitrogen oxides can be effectively suppressed, remaining ofuncombusted hydrocarbons can be effectively suppressed.

The “physical quantity directly indicative of exhaust substances”measured by the measuring unit 25 is, for example, the concentration ormass of the exhaust substance. In this case, for example, the measuringunit 25 is a NOx sensor that detects the concentration of nitrogen oxide(NOx). The unit of concentration is, for example, % or ppm. The unit ofmass is, for example, g or kg. When the exhaust substance is soot, themeasuring unit 25 is, for example, a soot sensor that detects sootvolume.

The control unit 211 controls the subsequent injection by the liquidfuel injecting unit 66 based on the physical quantity that directlyshows the exhaust substance. For example, the control unit 211 cancontrol at least one of the followings based on the physical quantitythat directly shows the exhaust substance: the injection volume ofliquid fuel in the subsequent injection, the timing of the subsequentinjection, and the number of subsequent injections.

The “physical quantity that indirectly shows the exhaust substance”measured by the measuring unit 25 is, for example, the in-cylinderpressure (internal pressure of the combustion chamber 71), thetemperature of the exhaust gas, or the air excess ratio λ. In this case,for example, the measuring unit 25 is a pressure sensor or a temperaturesensor. Alternatively, the measuring unit 25 is an O₂ sensor, a λsensor, or an A/F sensor each for detecting the air excess ratio λ.

The control unit 211 estimates the combustion state of gas fuel based onphysical quantity that indirectly shows the exhaust substance. Then, thecontrol unit 211 controls the subsequent injection by the liquid fuelinjecting unit 66 based on the estimated result of the combustion stateof gas fuel. For example, the control unit 211 controls at least one ofthe followings based on the estimated result of the combustion state ofthe gas fuel: the injection volume of liquid fuel in the subsequentinjection, the timing of the subsequent injection, and the number ofsubsequent injections.

When the measuring unit 25 measures the temperature of the exhaust gas,the control unit 211 monitors the temperature of the exhaust gas therebyto control the temperature of the exhaust gas to be equal to or lessthan a threshold temperature. As a result, an excessive increase inexhaust gas temperature during the subsequent injection can besuppressed.

Next, the gas fuel combustion method according to the modified exampleis described with reference to FIGS. 2 and 9 . FIG. 9 is a flowchartshowing the gas fuel combustion method according to the modifiedexample. The gas fuel combustion method is performed by the enginesystem 100 shown in FIG. 2 . As shown in FIG. 9 , the gas fuelcombustion method includes steps S11 through S15.

First, in step S11, the control unit 211 of the engine system 100controls the liquid fuel injecting unit 66 in a manner to perform themain injection. As a result, the liquid fuel injecting unit 66 performsthe main injection. As a result, gas fuel is combusted.

Next, in step S12, the control unit 211 controls the liquid fuelinjecting unit 66 in a manner to perform the subsequent injection. As aresult, the liquid fuel injecting unit 66 performs the subsequentinjection. Specifically, after the flame propagation after the ignitionof gas fuel is ended, the liquid fuel injecting unit 66 executes theinjection of liquid fuel.

Next, in step S13, it is determined whether or not the predeterminednumber of combustion cycles have been performed.

When it is determined in step S13 that the predetermined number ofcombustion cycles have not been performed (No), the process proceeds tostep S11.

Meanwhile, when it is determined in step S13 that the predeterminednumber of combustion cycles have been performed (Yes), the processproceeds to step S14.

Next, in step S14, the control unit 211 obtains measurement data fromthe measuring unit 25. The measurement data show a measured result ofthe physical quantity that directly or indirectly shows the emissionsubstance.

Next, in step S15, the control unit 211 determines, based on themeasured data, a control parameter for the subsequent injection by theliquid fuel injecting unit 66. The control parameter includes at leastone of the followings: the number of subsequent injections, theinjection volume of liquid fuel in the subsequent injection, and thetiming of the subsequent injection.

After step S15, the process proceeds to step S11. Next, in step S11, thecontrol unit 211 controls the liquid fuel injecting unit 66 in a mannerto perform the main injection. Next, in step S12, the control unit 211controls the liquid fuel injecting unit 66 in a manner to perform thesubsequent injection according to the control parameter determined inprevious step S15.

In this case, too, the injection (subsequent injection) of liquid fuelis performed after the flame propagation after the ignition of gas fuelis ended. Thereafter, step S13 through step S15 are executed. Further,repeating the combustion cycle repeats steps S11 through S15.

As described above with reference to FIG. 9 , the modified examplerepeats the combustion cycle, performing the main injection and thesubsequent injection for each combustion cycle.

The present invention will be specifically described with reference tothe examples, but the present invention is not limited to the followingexamples. [Examples]

Referring to FIGS. 1 and 10 , examples 1 to 6 and a comparative exampleof the present invention are described. In the examples 1 to 6, theengine system 100 shown in FIG. 1 was used. In the examples 1 to 6, thesubsequent injection is performed after the flame propagation after thegas fuel ignition was ended.

Meanwhile, in the comparative example, no subsequent injection wasperformed. In the examples 1 to 6 and the comparative example, naturalgas vaporized from liquefied natural gas was used as the gas fuel.

FIG. 10A is a graph showing the remaining volume of uncombustedhydrocarbons according to the examples 1 to 6 of the present invention.The ordinate shows the remaining volume of uncombusted hydrocarbons(ppmC1). FIG. 10B is a graph showing the generated volume of nitrogenoxides according to the examples 1 to 6 of the present invention. Theordinate shows the generated volume (ppm) of nitrogen oxides. In FIGS.10A and 10B, the abscissa shows the timing of the subsequent injectionby the crank angle (deg. aTDC). The crank angle of 0 degree shows thetop dead center of the piston 68.

In FIG. 10A, the dashed line L1 shows the remaining volume (L1) ofuncombusted hydrocarbons that is seen when no subsequent injection isperformed (main injection only, comparative example). A dot Da,connected by a solid line, shows the remaining volume (hereafter Da) ofuncombusted hydrocarbons that is seen when the subsequent injection isperformed in the example 1. A dot Db, connected by a dashed line, showsthe remaining volume (hereafter Db) of uncombusted hydrocarbons that isseen when the subsequent injection is performed in the example 2. A dotDc, connected by a single dotted line, shows the remaining volume(hereafter Dc) of uncombusted hydrocarbons that is seen when thesubsequent injection is performed in the example 3.

In FIG. 10B, the dashed line L2 shows the generated volume (hereafterL2) of nitrogen oxides that is seen when no subsequent injection isperformed (main injection only, comparative example). A dot DA connectedby a solid line shows the generated volume (hereafter DA) of nitrogenoxides that is seen when the subsequent injection is performed in theexample 1. A dot DB connected by a dashed line shows the generatedvolume (hereafter DB) of nitrogen oxides that is seen when thesubsequent injection is performed in the example 2. The dots DCconnected by a single dotted line shows the generated volume (hereafterDC) of nitrogen oxides that is seen when the subsequent injection isperformed in the example 3.

As shown in FIGS. 10A and 10B, in each of the examples 1 to 3, thesubsequent injection was performed at crank angles of 30°, 50°, 70°, and90°. In the example 1 (remaining volume Da, generated volume DA), theinjection volume of the subsequent injection was “20 mm³”. In theexample 2 (remaining volume Db, generated volume DB), the injectionvolume of the subsequent injection was “40 mm³”. In the example 3(remaining volume Dc, generated volume DC), the injection volume of thesubsequent injection was “60 mm³”. In examples 1 to 3 and theComparative example, the mixed gas supplied to the combustion chamber 71was in an effective lean state. Specifically, λ=about 1.81. In theexamples 1 to 3 and the comparative example, the specified range of theair excess ratio λ that defines the effective lean state shows between1.80 and 2.00, both inclusive.

As shown in FIG. 10A, in the examples 1 to 3, the remaining volumes Dato Dc of uncombusted hydrocarbons that are seen when the subsequentinjection is performed were less than the remaining volume L1 ofuncombusted hydrocarbons (comparative example) that is seen when thesubsequent injection is not performed. In particular, in the examples 1to 3, the larger the crank angle at which the subsequent injection isperformed, the smaller the remaining volumes Da to Dc of uncombustedhydrocarbons. In other words, in the examples 1 to 3, the later thetiming of the subsequent injection after the flame propagation after theignition of the gas fuel was ended, the smaller the remaining volumes Dato Dc of uncombusted hydrocarbons. From the examples 1 to 3, the largerthe injection volume of the subsequent injection, the smaller theremaining volumes Da to Dc of uncombusted hydrocarbons.

As shown in FIG. 10B, in examples 1 to 3, the generated volumes DA to DCof nitrogen oxides that are seen when the subsequent injection isperformed were less than the generated volume L2 of nitrogen oxides(Comparative example) when the subsequent injection was not performed.In particular, in the examples 1 to 3, the smaller the crank angle atwhich the subsequent injection is performed, the smaller the generatedvolumes DA to DC of nitrogen oxides. In other words, in the examples 1to 3, the earlier the timing of the subsequent injection after the flamepropagation after the ignition of the gas fuel is ended, the smaller thegenerated volumes DA to DC of nitrogen oxides. From the examples 1 to 3,the larger the injection volume of the subsequent injection, the lessthe generated volumes DA to DC of nitrogen oxides.

As shown in FIGS. 10A and 10B above, in the examples 1 to 3, theremaining volumes Da to Dc of uncombusted hydrocarbons and the generatedvolumes DA to DC of nitrogen oxides could be reduced compared to thecomparative example. In the example 1, the larger the reduction width(=L1−Da) of the remaining volume Da of uncombusted hydrocarbons, thesmaller the reduction width (=L2−DA) of the generated volume DA ofnitrogen oxides. In the example 1, the larger the reduction width of thegenerated volume DA of nitrogen oxides, the smaller the reduction widthof the remaining volume Da of uncombusted hydrocarbons. These pointswere also true for the examples 2 and 3.

Next, referring to FIG. 10 continuously, examples 4 to 6 different inair excess ratio λ from examples 1 to 3 are described.

In FIG. 10A, a dot da shown by a white circle shows the remaining volume(hereafter da) of uncombusted hydrocarbons that is seen when thesubsequent injection is performed in the example 4. A dot db shown by awhite triangle shows the remaining volume (hereafter db) of uncombustedhydrocarbons that is seen when the subsequent injection is performed inthe example 5. The dot dc shown as a white square shows the remainingvolume (hereafter dc) of uncombusted hydrocarbons that is seen when thesubsequent injection is performed in the example 6.

In FIG. 10B, a dot dA shown by a white circle shows the generated volume(hereafter dA) of nitrogen oxides that is seen when the subsequentinjection is performed in the example 4. A dot dB shown by a whitetriangle shows the generated volume (hereafter dB) of nitrogen oxidesthat is seen when the subsequent injection is performed in the example5. A dot dC shown in a white square shows the generated volume(hereafter dC) of nitrogen oxides that is seen when the subsequentinjection is performed in the example 6.

As shown in FIGS. 10A and 10B, in the examples 4 to 6 (remaining volumesda to dc, and generated volumes dA to dC), the subsequent injection wasperformed at the crank angle of 30°. In the examples 4 to 6, the mixedgas supplied to the combustion chamber 71 was in a lean state richerthan the effective lean state. Specifically, λ=1.76. In the examples 4to 6, the specified range of the air excess ratio λ that defines theeffective lean state shows between 1.80 and 2.00, both inclusive. Thelean state shows a state where the air excess ratio λ is greater than“1”. Other conditions in the example 4 were the same as in the example1, other conditions in the example 5 were the same as in the example 2,and other conditions in the example 6 were the same as in the example 3.

As shown in FIG. 10A, as a result of the comparison between the examples4 to 6 (remaining volumes da to dc) and the examples 1 to 3 (remainingvolumes Da to Dc) at the same crank angle (=30 degrees), the remainingvolumes da to dc of uncombusted hydrocarbons in “the lean state richerthan the effective lean state” were less than the remaining volumes Dato Dc of uncombusted hydrocarbons in the effective lean state. In otherwords, reducing the air excess ratio λ (air-fuel ratio) reduced theremaining volumes da to dc of uncombusted hydrocarbons.

As shown in FIG. 10B, as a result of the comparison between the examples4 to 6 (generated volumes dA to dC) and the examples 1 to 3 (generatedvolumes DA to DC) at the same crank angle (=30 degrees), the generatedvolumes dA to dC of nitrogen oxides in “the lean state richer than theeffective lean state” were more than the generated volumes DA to DC ofnitrogen oxides in the effective lean state. In other words, reducingthe air excess ratio λ (air-fuel ratio) increased the generated volumesdA to dC of nitrogen oxides.

The embodiments and examples of the present invention have beendescribed with reference to the drawings. However, the present inventionis not limited to the embodiments and the examples described above, andcan be performed in various aspects in a range without departing fromits spirit. The plurality of components disclosed in the aboveembodiments may be modified as appropriate. For example, one of all thecomponents shown in one embodiment may be added to the other embodimentor some components of all the components shown in one embodiment may beremoved from the embodiment.

The drawings schematically show each component as a main subject as soto facilitate understanding of the present invention, and the thickness,length, quantity, spacing, and so on of each shown component is, as thecase may be, different from the actual ones due to the convenience ofthe drawings. Further, it is needless to say that the configuration ofeach component shown in the above embodiments is merely an example andis not particularly limited, and various modifications may be madewithout substantially departing from the effect of the presentinvention.

The engine system 100 described with reference to FIG. 1 had only a gasfuel mode. The gas fuel mode is a mode in which mechanical work isobtained by combustion of gas fuel. However, the engine system 100 mayhave a gas fuel mode and a liquid fuel mode. The liquid fuel mode is amode in which mechanical work is obtained by combustion of liquid fuel.In this case, in addition to the liquid fuel injecting unit 66 thatinjects liquid fuel for ignition, the engine system 100 is equipped withanother liquid fuel injecting unit that injects, to the combustionchamber 71, the liquid fuel for obtaining mechanical work throughcombustion. The engine system 100 may also have a mixed combustion mode.The mixed combustion mode is a mode in which both gaseous fuel andliquid fuel are combusted at substantially the same time thereby toobtain mechanical work.

INDUSTRIAL APPLICABILITY

The present invention relates to an engine system and a gas fuelcombustion method, and has industrial applicability.

REFERENCE SIGNS LIST

-   1 engine-   21 engine control unit-   25 measuring unit-   65 gas fuel supply unit-   66 liquid fuel injecting unit-   68 piston-   71 combustion chamber-   100 engine system-   211 control unit

1. An engine system that has a combustion chamber to which air and a gas fuel are supplied, and combusts the gas fuel, the engine system comprising: a liquid fuel injecting unit configured to inject a liquid fuel thereby to ignite the gas fuel; and a control unit configured to control the liquid fuel injecting unit, wherein the control unit is configured to control the liquid fuel injecting unit so that injection of the liquid fuel is performed after a flame propagation after ignition of the gas fuel is ended.
 2. The engine system according to claim 1, wherein the control unit configured to control the liquid fuel injecting unit so that the injection of the liquid fuel after the ending of the flame propagation is started within a former half of a combustion period of the gas fuel.
 3. The engine system according to claim 1, wherein: a plurality of combustion cycles are performed in a predetermined period, the liquid fuel injecting unit is configured to, for each of the combustion cycles, perform the ignition of the gas fuel, and the injection of the liquid fuel after the ending of the flame propagation, and the control unit is configured to, within the predetermined period, prohibit an injection volume of the liquid fuel from increasing based on the ignition of the gas fuel.
 4. The engine system according to claim 3, wherein: the increasing in the injection volume of the liquid fuel based on the ignition of the gas fuel refers to that the injection volume of the liquid fuel increases according to an increase in a load on an engine, and the predetermined period is 10 seconds.
 5. The engine system according to claim 1, further comprising: a measuring unit configured to measure a physical quantity that directly or indirectly shows an exhaust substance generated based on combustion of the gas fuel, and wherein the control unit is configured to, based on a measurement result by the measuring unit, control injection of the liquid fuel after the ending of the flame propagation.
 6. The engine system according to claim 1, wherein the control unit the control unit is configure to control an air excess ratio in the combustion chamber according to a condition when the liquid fuel is injected after the ending of the flame propagation.
 7. The engine system according to claim 1, further comprising: a piston configured to move in the combustion chamber, wherein an injection angle of the liquid fuel relative to a direction of movement of the piston is between 30 degrees and 65 degrees, both inclusive.
 8. The engine system according to claim 1, wherein the control unit is configured to control the liquid fuel injecting unit so that the injection of the liquid fuel after the ending of the flame propagation is performed after a top dead center of a piston at a crank angle in a range of 30 degrees or more and less than 60 degrees.
 9. The engine system according to claim 1, wherein the control unit is configured to change at least one of following: a number of injecting operations of the liquid fuel after the ending of flame propagation, an injection volume of the liquid fuel after the ending of the flame propagation, or an injection timing of the liquid fuel after the ending of the flame propagation, and thereby, adjust a volume of an exhaust substance from an engine.
 10. A gas fuel combustion method in an engine in which air and a gas fuel are supplied to a combustion chamber thereby to combust the gas fuel, the method comprising: a step of injecting a liquid fuel thereby to ignite the gas fuel; and a step of performing an injecting of the liquid fuel after a flame propagation after the igniting of the gas fuel is ended.
 11. The gas fuel combustion method according to claim 10, wherein the injecting of the liquid fuel after the ending of the flame propagation is started within a former half of a combustion period of the gas fuel.
 12. The gas fuel combustion method according to claim 10, wherein: a plurality of combustion cycles are performed in a predetermined period, for each of the combustion cycles, the igniting of the gas fuel, and the injecting of the liquid fuel after the ending of the flame propagation are performed; and within the predetermined period, an injection volume of the liquid fuel is prohibited from increasing at the igniting of the gas fuel.
 13. The gas fuel combustion method according to claim 12, wherein: the increasing in the injection volume of the liquid fuel at the igniting of the gas fuel refers to that the injection volume of the liquid fuel increases according to an increase in a load on the engine, and the predetermined period is 10 seconds.
 14. The gas fuel combustion method according to claim 10, wherein the liquid fuel's injecting after the ending of the flame propagation is performed after a top dead center of a piston at a crank angle in a range of 30 degrees or more and less than 60 degrees.
 15. The gas fuel combustion method according to claim 10, further comprising: changing: a number of injecting operations of the liquid fuel after the ending of flame propagation, an injection volume of the liquid fuel after the ending of the flame propagation, or an injection timing of the liquid fuel after the ending of the flame propagation, and thereby, a volume of an exhaust substance from the engine is adjusted. 